WASHINGTON, May 24 (Xinhua) -- U.S. Democratic front-runner Hillary Clinton continues to have a slight image advantage over presumptive Republican nominee Donald Trump among Americans, says a Gallup poll released Tuesday.

So far this month, 40 percent of Americans hold a favorable view of Clinton, while 54 percent have an unfavorable opinion. For Trump, Americans' views are 33 percent favorable and 60 percent unfavorable, according to the poll.

Overall, Clinton's image has gradually worsened over the course of the campaign since Gallup began tracking the candidates' images 10 months ago.

Meanwhile, Trump's image has also worsened, reaching a low point of -35 net favorable in March, although it improved this month to -27.

The poll comes at a time when the two candidates are running neck-in-neck in terms of which candidate will clinch more votes in November's general election.

While Clinton was several months ago considered a shoo-in to win the Democratic nomination, rival Senator Bernie Sanders has put up a tough fight in the primaries, winning several states.

At the same time, while Trump has galvanized rank-and-file Republican voters like perhaps no candidate in recent memory, his negative ratings are high.

The uptick in Trump's overall image in recent weeks results from his popularity increasing among Republicans. With Trump essentially securing the nomination, many who supported other Republican candidates are now supporting Trump, Gallup said.

Trump lost a little ground among Republicans in February and March, but his current net favorable rating among Republicans, at +33, is his highest monthly average yet within his own party.

Clinton's image among Democrats remains more positive than Trump's is among Republicans, but her image -- unlike his -- has not yet recovered from a slow slide downward, Gallup said.

Clinton's image among Democrats will likely improve when Sanders officially drops out of the race or when he acknowledges her as the presumptive nominee, although that may not occur until late July at the Democratic National Convention in Philadelphia.

All major presidential candidates in recent elections have ended their campaigns with favorable ratings at 46 percent or higher, with many well into majority territory.

A key element of this 2016 election to watch will be the degree to which Clinton and Trump manage to raise their favorable ratings, or if their images remain as negative as they are now going into the fall, Gallup said.