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Representative Tammy Duckworth, a Democrat, speaking during a rally at the Illinois State Fair in Springfield on Aug. 20.Credit Seth Perlman/Associated Press

WASHINGTON — While the nation has been preoccupied with cacophonous presidential primary battles, the fight over control of the Senate has been quietly falling into place.

Republican incumbents, who have the daunting task of defending 22 Senate seats in 2016, with about half in competitive states, appear to have snuffed out the ideological wars that have bedeviled them in the last four election cycles; there is little evidence that Tea Party-endorsed challengers are targeting sitting senators.

For now, the party’s fight for ideological purity has shifted to the race for the White House, and to within the halls of Congress. A dearth of tough primaries will most likely leave incumbents’ checkbooks and stamina intact for the general election.

A primary drama of sorts has emerged in the Democratic Party, however, where battle lines are drawn less between dogmas than between establishment-chosen candidates and those trying to bust through the Beltway gatekeepers.

In Illinois, for instance, Representative Tammy Duckworth, a Democrat favored to take on the deeply vulnerable Republican senator, Mark S. Kirk, is feeling pressure from supporters of Andrea Zopp, former president of the Chicago Urban League. This month, Ms. Zopp told reporters, “Washington insiders won’t be telling voters of Cook County who they should choose.”

In Florida, the Democrats’ chosen candidate, Representative Patrick Murphy, is battling Alan Grayson, Mr. Murphy’s House colleague and a liberal firebrand.

And if Senator Mitch McConnell of Kentucky is to be more than just a short-term majority leader, it is up to Republican senators who blew into Washington with their party’s 2010 tsunami — Kelly Ayotte of New Hampshire, Patrick J. Toomey of Pennsylvania, Ron Johnson of Wisconsin and Rob Portman of Ohio. They must prove they can get re-elected in a presidential year in purple states.

It is still early. Democrats have no preferred candidate in New Hampshire and North Carolina, and Republicans lack one in Colorado.

But the Democrats’ goal of recapturing the Senate, which would require gaining four or five seats, depending on the results of the presidential race, and the Republicans’ hope of fending them off will almost certainly be tied to the fate of each party’s presidential candidate and how voters assess the legacy of President Obama.

“One big uncertainty with this cycle is how this president leaves office,” said Nathan L. Gonzales, the editor of The Rothenberg & Gonzales Political Report, a nonpartisan newsletter. “Do voters want change or status quo, based on the last eight years?”

After several years of watching Republicans maul one another in primary battles, Democrats now have a few skirmishes of their own, partly because of their myriad opportunities. To topple Mr. Kirk, the Illinois senator, Democrats have largely lined up behind Ms. Duckworth, a disabled Iraq war veteran. But recently, Democrats in Cook County declined to endorse her after African-Americans criticized the national party’s early dismissal of Ms. Zopp, who is black.

“In a primary election, the African-American vote is a very significant vote,” said Representative Danny K. Davis, a longtime Chicago Democrat. “There are African-Americans who felt they were not consulted, or overlooked, or taken for granted.”

There are also primary contests in Ohio — where former Gov. Ted Strickland, a Democrat, is widely expected to prevail — as well as in Florida, Pennsylvania and a handful of other states.

While some Democrats view their primaries as less consequential because they are in states that will not be very competitive in the general election, that may not be true in a state like Florida, where Mr. Grayson is to the left of many Democrats.

“Democrats living in the Panhandle are certainly more conservative, and they are high-turnout voters,” said Susan MacManus, a University of South Florida political scientist. “So even though they make up a small share of the electorate, their preferences are viewed seriously by party leaders since the last three elections in Florida have been decided by 1 percent or less.”

In Pennsylvania, establishment Democrats are viewing Mr. Toomey with the longing of a lion staring at a wildebeest. But they were not excited when Joe Sestak, who narrowly lost to Mr. Toomey in 2010, said he would try again; they have set their hopes on Katie McGinty, who has served under Democrats at the federal and state level.

That primary is also less about political beliefs — most expect the two would vote the same in Congress — than it is about the choice of party regulars versus an outsider.

“It’s not like this is groundswell from the bottom up to find an alternative,” said G. Terry Madonna, the director of the Franklin & Marshall College Poll, which recently found Mr. Toomey leading Mr. Sestak by 12 percentage points, and Ms. McGinty by seven points. “It was top down.”

Mr. Toomey is closely associated with a failed Senate gun-safety bill that is similar to current Pennsylvania law, and he rarely utters the words Club for Growth, the conservative group he once led, one that often tries to take down sitting members of Congress.

For many Republicans who won in 2010 with a wave of Tea Party candidates, the challenge will be to tone down conservative speech and to portray themselves as bipartisan problem-solvers.

In a telling late-night budget vote this year, several Republican incumbents, including Ms. Ayotte, Mr. Portman, Mr. Johnson and Senator Richard M. Burr of North Carolina, voted in favor of a Democratic amendment that would give married same-sex couples access to Social Security and Department of Veterans Affairs benefits.

Ms. Ayotte, who early in her Senate career aligned herself with some of the most conservative senators on national security issues, has calibrated her positions lately. Her overall rating from the American Conservative Union — one of the oldest conservative groups in the nation — is 75.5, but her score for this year is 63 and on a downward trend.

“She has made a conscious political effort to move her voting record a little bit toward the center,” said Paul Hodes, a former House member whom Ms. Ayotte defeated in 2010. “She is a champion of vets. That plays really well here, and she gets very good press in New Hampshire for doing things the eccentric cantankerous people here like.”

In Ohio, Mr. Portman has spent the August recess calling attention to bipartisan bills like his work force development legislation with Senator Michael Bennet, Democrat of Colorado. He is also meeting with union and steel workers to talk about his trade enforcement.

“My message is that if you rehire me, it will be someone who gets things done, not someone who tries to score partisan points,” Mr. Portman said in a telephone interview from Ohio.

But the fate of candidates in swing states will be heavily influenced by the fate of each party’s White House candidate.

First, there are the additional campaign resources in a presidential year. “I got zero help from national party when there was no presidential campaign,” Mr. Hodes said.

Republicans in several states will also need to rely on split-ticket voting to prevail.

“Seven United States Senate candidates since World War II have won election while the presidential candidate from the other state carried it the other way,” Mr. Madonna said. “But with heightened polarization partisanship, that may not be possible anymore.”