Hillary Clinton’s favorability level slipped a bit this week, dropping to its lowest since 2003. The front-runner for the Democratic Party's presidential nomination also ceded a little ground to her potential 2016 primary rivals, according to a poll released Tuesday by CNN. After Clinton, Democrats said they were next most likely to vote for Vice President Joe Biden, although he was still leagues behind her, at 14 percent compared to Clinton’s 60, and may not run at all. Of those actually in the Democratic race, Vermont Sen. Bernie Sanders had an uptick in the month since he announced, taking 10 percent of the likely primary vote.

Clinton's favorability numbers, at 46 percent, were the lowest recorded since March 2003, when she was a senator from New York and came in at 45 percent. Her unfavorable numbers were the highest they've been since March 2001, two months after she and her husband left the White House. Fifty percent of respondents had an unfavorable view of the Democratic front-runner last month. 

While she is still very much in the lead, the declining numbers show some amount of vulnerability to candidates entering the race against the former secretary of state. Sanders, who didn’t announce his presidential bid until late April, after the previous CNN/ORC poll, doubled his likely vote tally. Biden, who has not signaled an intention to run, has remained relatively constant over the last few polls.

The other declared Democratic candidate, former Maryland Gov. Martin O’Malley, barely registered in the poll with 1 percent, just behind former Virginia Sen. Jim Webb's 2 percent. Former Rhode Island Gov. Lincoln Chafee, who is expected to jump in the race Wednesday, was not a choice of voters at all. He was a Republican U.S. senator and was elected governor as an independent.

On the Republican side, Sen. Marco Rubio of Florida took the lead for the first time, moving up to 14 percent along a steady rising path. Former Florida Gov. Jeb Bush dropped four points to 13 percent, just behind Rubio. Following those two were former Arkansas Gov. Mike Huckabee and Wisconsin Gov. Scott Walker, each with 10 percent.

In matchups between Clinton and five of the top Republicans chosen by voters, she retained an edge over all of them. The closest Republican challenger was Sen. Rand Paul of Kentucky, who trailed her by just 1 percent. In the Republican field, Paul tied with Sen. Ted Cruz of Texas for fifth place.

The poll consisted of interviews with 1,025 American adults by landline and cell phone. It was conducted Friday and Saturday and has a margin of error of plus or minus three percentage points.

Clinton has slipped with voters a bit in another poll as well. In a Washington Post/ABC poll, she also received her lowest marks in years, according to Politico.