Washington(CNN) Hillary Clinton would beat Donald Trump in a general election match-up if the election were held today, a new poll shows.

Clinton beats Trump, 49% to 39%, head-to-head. She would be neck-and-neck with other GOP contenders -- including former tech CEO Carly Fiorina (45% to Clinton's 44%), retired neurosurgeon Ben Carson (46% to Clinton's 45%), former Florida Gov. Jeb Bush (44% to Clinton's 45%).

But if Vice President Joe Biden runs for president, he'd enter the race as the most popular presidential candidate, according to the NBC News/Wall Street Journal survey.

Clinton's favorability rating is underwater: 39% of voters view her favorably, while 47% say they see the former secretary of state unfavorably.

But Trump's problem is much bigger. He's seen favorably by 25% of the general electorate, compared to unfavorably by 58%.

Biden, meanwhile, is the most popular potential candidate -- in part because he hasn't taken the shots that he'd be hit with if he were to enter the race. He's viewed favorably by 40% compared to unfavorably by 28%.

Another Democratic candidate, Vermont Sen. Bernie Sanders, is also popular, at 32% favorable versus 22% unfavorable.

The most popular Republican is Carson, whose 29% favorability rating outpaces his 21% unfavorability rating. He's followed by Fiorina (27% to 20%).

The poll shows Clinton with a 7-point lead among national Democrats, at 42% to Sanders' 35% and Biden's 17% and

Trump leads among Republicans, at 21% to Carson's 20%, with Fiorina and Florida Sen. Marco Rubio tying at 11%, followed by Bush's 7%, Ohio Gov. John Kasich's 6% and Texas Sen. Ted Cruz's 5%.

Kentucky Sen. Rand Paul and New Jersey Gov. Chris Christie are at 3% each, while former Arkansas Gov. Mike Huckabee is at 2%.

The general election poll was conducted September 20-24, surveying a total of 1,000 adults for a margin of error of plus or minus 3.1 percentage points; the Democratic poll surveyed 256 primary voters for a margin of error of plus or minus 6.13 percentage points; and the Republican poll surveyed 230 primary voters for a margin of error of plus or minus 6.46 percentage points.