Has Hillary Clinton's standing with the public declined sharply in recent weeks? Evidence that it has comes from Quinnipiac polls in three key swings states: Colorado, Iowa and Virginia. In each state, Clinton is matched against three relatively well-known Republican candidates: Jeb Bush, Scott Walker and Marco Rubio. In each state she trails all three. No candidate in any of these nine pairings reaches 50 percent. Bush comes in slightly lower, at 41 and 42 percent, than Walker (43-47 percent) and Rubio (43-46 percent).

But the real story is the sag in Clinton's numbers. Quinnipiac finds that her favorability rating is just 35 percent in Colorado, 33 percent in Iowa and 41 percent in Virginia. In previous Quinnipiac and other polls in Iowa and Colorado, Clinton was running above 40 percent against these Republicans; in this Quinnipiac poll she is running significantly under 40 percent — a sharp and significant drop.

Clinton's numbers against these Republicans are not down quite so much in Virginia, where they hover around 40 percent. One reason may be that Virginia has a much higher percentage of black voters (20 percent in the 2012 exit poll) than the other two states and that black voters may be more willing to support her than non-black 2012 Obama voters. Note that a similar effect is not apparent in Colorado, where the 2012 exit poll said 14 percent of the voters were Hispanics and that they voted 75 percent for Barack Obama.

In 2012, Barack Obama won 51 percent of the vote in Colorado, 52 percent in Iowa and 51 percent in Virginia, the state which came closest to the national percentages in both 2008 and 2012. The three states together have 28 electoral votes, and this poll suggests that Hillary Clinton, as of now at least, would have difficulty carrying them. If Clinton's standing has similarly declined in Florida (29 electoral votes, 50 percent Obama) and Ohio (18 electoral votes, 51 percent Obama), that would plunge the Democratic electoral vote count below the majority of 270.

Of course this is just one poll (or set of polls), and any trend in these three states may not extend to other states. National polls matching Clinton against Bush, Walker and Rubio do not yet show any similar decline in her standing. So caution is in order. But Democrats who have been reluctantly willing to accede to Clinton's nomination on the grounds that she is a sure general election winner might well have some second thoughts if we start to see similar results in other polls.