WASHINGTON - Hillary Rodham Clinton's troubles are costing her politically, as potential Republican presidential rivals have inched closer to her in 2016 matchups, a new McClatchy-Marist poll found Friday.

The former secretary of state fell below the crucial 50 percent level of support in one-on-one matchups against Jeb Bush, Marco Rubio and Scott Walker, and she was barely above that benchmark against Rand Paul, Rick Perry and Ted Cruz.

None have formally declared themselves candidates yet.

In the race for the Republican nomination, Bush, a former Florida governor, and Walker, the governor of Wisconsin, led the wide-open field. Walker has emerged as a favorite of conservatives, while Bush leads among moderates.

Their competitive showings against Clinton "may tap into some concerns voters have about her," said Lee Miringoff, director of the Marist Institute for Public Opinion in New York, which conducted the survey. "It gets us back to stuff people find unpleasant about the Clintons."

Clinton was first dogged by reports last month about millions of dollars her family's foundation has received from donors based in foreign countries. Monday, the second day of the polling, the New York Times reported that Clinton had used a personal e-mail server to conduct government business while secretary of state.

The e-mail furor dominated political news all week. Wednesday, a House panel that's investigating the 2012 attacks in Benghazi, Libya, subpoenaed relevant e-mails.

The survey found that Democrats continue to give Clinton strong support. Sixty percent of Democrats or Democratic-leaning independents named her as their pick for president. Vice President Biden, with 13 percent, and Sen. Elizabeth Warren of Massachusetts, at 12 percent, were far behind. Warren has said she's not interested in running.

Clinton got solid support from almost all segments of the Democratic electorate. Her biggest vulnerability was among men, where she won 52 percent backing. About two in three women supported her.

In head-to-head matchups with top Republicans, though, the plot line was very different.

Clinton led Walker, who has vaulted into the top tier of possible Republican contenders in recent weeks, by 48 percent to 44 percent. While Clinton won among moderates and liberals, Walker had a 70 percent to 26 percent advantage with conservatives.

Those conservatives have driven his emergence toward the front of the pack for the GOP nomination. The poll found Walker the runaway favorite among very conservative Republicans.

Bush scored well among Republican moderates, winning about one-fourth of their support in the GOP contest, well ahead of Walker and New Jersey Gov. Christie.

Bush is also faring better against Clinton. She topped him 49 percent to 42 percent, after beating him 53 percent to 40 percent in December.

Clinton also fell below 50 percent against Rubio, a U.S. senator from Florida, beating him 49 percent to 42 percent.

Against other Republicans, she was ahead 51 percent to 42 percent over Perry, a former Texas governor. That 9-point margin was down from December, when she led Perry by 58 percent to 37 percent.

She led 51 percent to 40 percent last week over Paul, a U.S. senator from Kentucky. Her 11-point margin was down slightly from December, when she led 54 percent to 40 percent.

And she led 53 percent to 39 percent last week over Cruz, a U.S. senator from Texas. That was essentially the same as her 54 percent to 39 percent lead in December.

Experts consider a drop below 50 percent a danger sign for well-known candidates. It suggests that more than half the electorate has judged them and is looking elsewhere. While the Republican candidates aren't well known, and it's very early in the nominating process, there appears to be an opening for a credible challenge to Clinton.

The survey of 1,253 adults was conducted Sunday through Wednesday by the Marist Poll, sponsored in partnership with McClatchy. Results are statistically significant within 2.8 percentage points.