WASHINGTON — As the administration struggles to put in place the final, complex piece of President Obama’s signature health care law, an endeavor on a scale not seen since Medicare’s creation nearly a half-century ago, Democrats are worried that major snags will be exploited by Republicans in next year’s midterm elections.

Many Democrats also want to see a more aggressive and visible president to push the law across the country. This week Mr. Obama is returning to the fray to an extent unseen since he signed the law in 2010, including a White House event on Friday to promote the law’s benefits for women, the first in a series of appearances for health care this year.

A number of health insurance changes have already taken place, but this fall, just as the 2014 election season heats up, is the deadline for introducing the law’s core feature: the insurance marketplaces, known as exchanges, where millions of uninsured Americans can buy coverage, with subsidies for many.

For the third time, Republicans are trying to make the law perhaps the biggest issue of the elections, and are preparing to exploit every problem that arises. After many unsuccessful efforts to repeal the law, the Republican-led House plans another vote soon. And Republican governors or legislatures in many states are balking at participating, leaving Washington responsible for the marketplaces.

“There are very few issues that are as personal and as tangible as health care, and the implementation of the law over the next year is going to reveal a lot of kinks, a lot of red tape, a lot of taxes, a lot of price increases and a lot of people forced into health care that they didn’t anticipate,” said Brad Dayspring, spokesman for the National Republican Senatorial Committee. “It’s going to be an issue that’s front and center for voters even in a more tangible way than it was in 2010.”

That year a conservative backlash against the new law helped Republicans take control of the House. But last year Mr. Obama was re-elected, and Democrats gained seats in Congress.

Democrats are worried about 2014 — a president’s party typically loses seats in midterm years — and some have gone public with concerns about the pace of carrying out the law. Senator Harry Reid of Nevada, the majority leader, told an interviewer last week that he agreed with a recent comment by Senator Max Baucus of Montana, a Democratic architect of the law, who said “a train wreck” could occur this fall if preparations fell short.

The White House has allayed some worries, with briefings for Democrats about their public education plans, including PowerPoint presentations that show areas with target populations down to the block level.

“There’s clearly some concern” among Democrats “that their constituents don’t yet have all facts on how it will work, and that Republicans are filling that vacuum with partisan talking points,” said Representative Steve Israel of New York, head of the House Democrats’ campaign committee. “And the administration must use every tool they have to get around the obstructions and make it work.”

Click here for the full story.