WASHINGTON — The Senate headed into its August recess on Wednesday without voting on a cybersecurity bill, adding it to a contentious to-do list for September that includes a push to disapprove the Iran nuclear deal and a spending fight mired in abortion politics.

The delay of the cybersecurity bill followed a pattern that has marked the Senate since the Republicans took control this year. Senator Mitch McConnell of Kentucky, the majority leader, has struggled to accelerate the legislative track while still permitting members of both parties to amend bills on the floor, as he has repeatedly promised to do.

In addition, Republicans’ return to the thorny abortion debate — this time by inserting a bill to defund Planned Parenthood into the calendar ahead of the cybersecurity measure — left too little time to finish the computer security legislation. Presidential politics has also frequently played a part, with four Republican senators set to participate in primary debates on Thursday.

Mr. McConnell tried this summer to attach the cybersecurity measure to a broader defense bill as a way to mousetrap Democrats into voting for it, but that effort failed. Mr. McConnell then went on to other legislative matters, including a major trade package. When the Senate finally turned to the cybersecurity bill, Mr. McConnell could not get an easy agreement on amendments, which further delayed the action, as did objections from some privacy advocates.

Negotiating amendment deals may have slowed legislation far more than Mr. McConnell anticipated when he took the Senate’s reins, even though he repeatedly used filibuster tactics based on amendment disagreements to block legislation when he was in the minority. He promised to deliver a different culture while in the majority, but getting consent among 100 senators is no easier for him than it was for his predecessor, Senator Harry Reid of Nevada.

This has sometimes produced dire results for Congress. A fight over changes to the Patriot Act — led by Senator Rand Paul of Kentucky, who wanted a long list of amendments — led to a short-term expiration of the law.

Few senators wanted to stick around beyond Wednesday, noting that the House was long gone. But senators in both parties said they feared that another cyberattack could hit the government or the private sector while they were home for the monthlong break. “There should be concern about that,” said Senator Roy Blunt, Republican of Missouri. Should there be a major attack in August, he said, voters could wonder “why government didn’t try to do something to prevent it.”

In April, the House passed a measure that would push companies to share access to their computer networks and records with federal investigators. Neither the House legislation nor the bill to be considered by the Senate would have prevented attacks such as the recent theft of the personal information of more than 20 million Americans from the Office of Personnel Management.

But the House measure was the legislative branch’s most aggressive response yet to a spate of private-sector computer attacks, including a major breach at Sony Pictures Entertainment, the exposure of credit card numbers of tens of thousands of Target customers, and the compromising of personal records of millions of people who did business with the health insurer Anthem.

“Our nation has unparalleled strength,” said Senator Susan Collins, Republican of Maine, who has been working for years to get a cybersecurity bill out of the Senate. “But cyberspace allows much weaker adversaries to target our people, our economy and our military.” She added, “We already know many of the steps necessary to reduce the likelihood of a cyber 9/11, yet many of these actions have not yet been taken in either the government or in the private sector.”

Republican control of Congress has not led to uniform policy prescriptions, and Republican infighting has often produced the biggest legislative headaches for Mr. McConnell. That was demonstrated last week when the House and the Senate could not agree on a long-term bill to pay for repairs to the nation’s crumbling roads and bridges, and punted for well over the 30th time with a three-month extension of financing for the program.

While Congress achieved many major accomplishments this year, including the final broad revisions to the Patriot Act, a new formula for paying doctors under Medicare, and two bills that may be reconciled to fix the No Child Left Behind law, other legislation continues to be hard-fought.

On the horizon is a budget battle that some Republicans said could lead to a government shutdown if Congress does not move to take away money from Planned Parenthood. Congressional leaders hope to at least pass a temporary funding measure before an address by Pope Francis to a joint meeting of Congress on Sept. 24.

“It takes a long time to do things in the Senate,” Mr. McConnell said to reporters last week.