– Democrats are finding that their path to keeping control of the U.S. Senate this year is getting bumpier.

At least four states where Democrats hold Senate seats that once were seen as fairly safe are now considered in play: Michigan, Iowa, Colorado and New Hampshire.

They join seven states with Democratic incumbents where analysts see decent bets for Republican pickups: Alaska, Arkansas, Louisiana, Montana, North Carolina, South Dakota and West Virginia. Republican presidential nominee Mitt Romney carried all seven in 2012.

The new four are now battlegrounds for the same reasons that plague Democrats elsewhere. The Affordable Care Act is detested in many circles. Anyone associated with Washington is often toxic. And popular Republicans who are running for other offices are often on the ballot.

“The common thread is that there’s a Democrat in the White House who’s not that popular,” said Kyle Kondik, the managing editor of Sabato’s Crystal Ball, a nonpartisan research group at the University of Virginia. “It wouldn’t be surprising if any of those states went Republican.’

Republicans also appear more motivated. “There’s a sense that a possible takeover of the Senate is real, and that will give a boost to the Republicans’ ability to thwart the president’s agenda,” said Chris Budzisz, the director of the Iowa-based Loras College Poll. He was speaking of Iowans, but the perception holds more broadly.

Republicans need a net gain of six seats for a Senate majority. Democrats are defending 21 seats, the GOP 15. Only two Republican-held seats are considered vulnerable. In Kentucky, if Senate Republican leader Mitch McConnell wins a primary in May as expected, he’ll be challenged by Secretary of State Alison Lundergan Grimes, a Democrat.

In Georgia, where Sen. Saxby Chambliss, R-Ga., is retiring, a big Republican field is seeking the nomination. The winner is expected to face well-funded Democrat Michelle Nunn.

The outlook in the new tossup Democratic-held states:

Michigan

Sen. Carl Levin, D-Mich., is retiring, and Rep. Gary Peters, D-Mich., is trying to take his place.

Republican former Secretary of State Terri Lynn Land, who’s also seeking the seat, led Peters by 6 percentage points this month in a poll by Mitchell Research.

New Hampshire

Sen. Jeanne Shaheen, a Democrat, has a long history of winning statewide races, first as a strategist and then as governor and senator.

And few thought Republican former Massachusetts U.S. Sen. Scott Brown would pose much of a challenge. Brown is now in the race, and a Granite State poll taken April 1-9 showed him 6 percentage points behind.

One of Shaheen’s problems: Those who have moved into the state from Massachusetts — about one-fourth of those surveyed — preferred Brown by 13 points.

Iowa

Sen. Tom Harkin, a Democrat, is retiring.

“It could very well become an opportunity for Republicans,” Budzisz said.

One reason is the strength of Gov. Terry Branstad, a highly popular Republican who’s running again.

Two other factors might determine the Senate winner: the eventual Republican nominee, who will be chosen later this year, and the campaign effectiveness of Democrat Bruce Braley, a four-term congressman.

Braley stumbled this year when he said Sen. Charles Grassley, a Republican, was “a farmer from Iowa who never went to law school, never practiced law.” Republicans complained that Braley was insulting farmers.

Colorado

Incumbent Mark Udall, a Democrat, had looked fairly safe until Rep. Cory Gardner, a popular Republican, entered the race in late February.

“It’s definitely a race,” said Denver-based consultant Floyd Ciruli. Udall’s ties to Obama, and his vote for the health care plan, hurt.

Obama won the state in 2012 with 51.4 percent of the vote, but this year the enthusiasm is on the Republican side, where partisans want to send a message to incumbents.

“People think Washington is the pits,” Ciruli said.

Democrats will try to paint Gardner as an out-of-touch conservative. Gardner scored an 84 rating from the American Conservative Union last year, slightly above Republicans’ 76.48 percent average. “He’s far out of step with mainstream Colorado voters,” Barasky said.

Could be, but Ciruli had a different take: “It’s a lousy year to be a Democrat.”