It's become a common talking point for "Never Trump" Republicans seeking to defeat President Trump in the 2020 GOP primaries: Polls have found that a substantial number, maybe 40 percent, of Republican voters say they would be open to a primary challenge to the president. Those surveys, the "Never Trumpers" argue, show that Trump is particularly weak and vulnerable to a serious primary challenge.

The problem is that it is common for voters to say they are open to a primary challenge to the president of their own party. It happened to former President Barack Obama in 2010-2011, before Obama went on to win re-election. It also happened to Bill Clinton in 1994-1995, before Clinton went on to win re-election. It did not happen to George W. Bush in 2002-2003, but that was because of strong Republican support for a wartime president.

In other words, today's poll results about President Trump are not at all unusual.

Talk about a primary challenge to Obama intensified around the 2010 midterms, when Democrats lost 63 seats in the House in an election Obama conceded was a "shellacking." The idea that Obama might face a challenge had been around since his divisive 2008 primary battle with Hillary Clinton. Democratic strategist Ed Kilgore wrote in the New Republic that by 2010 those tensions had "gotten worse with the rise of the 'angry left,' which thinks Obama has been too eager to compromise with Wall Street and Republicans, and considers itself the representative of the Democratic base."

With midterm campaigning looking bad for Democrats, polls indicated voter openness to an Obama challenge. In a September 2010 Gallup survey, 37 percent of Democrats said they would support a Hillary Clinton primary challenge to Obama, while 52 percent said they would support Obama, and 10 percent were undecided.

In late October 2010, an Associated Press poll found that 47 percent of Democrats said Obama "should be challenged" in a party primary. That included 29 percent of Democrats who said they originally supported Obama in the 2008 primary battle.

In late November 2010, after the midterm loss, a Marist poll asked, "Do you want another Democrat to challenge Barack Obama for the Democratic nomination for president in the 2012 primaries?" Forty-one percent of Democrats said yes, 45 percent when Democratic-leaning independents were included.

That same month, November, a Pew poll found that 38 percent of Democrats and Democratic leaners would like to see an Obama challenge.

In January 2011, an AP-GfK poll found that 44 percent of Democrats and Democratic-leaning independents said they would "like to see Barack Obama face a serious challenge in the 2012 presidential primaries from another Democratic candidate."

The specter of a primary challenge was much discussed among Democrats in those months, with prominent commentators anxious to knock it down and defend the president. "One thing we know is, if you have a primary challenge, it is almost lethal to the incumbent," MSNBC's Chris Matthews said in December 2010.

By August 2011, Pew reported that the percentage of Democrats who wanted to see Obama challenged had fallen a bit, from 38 percent to 32 percent. But that lower number was still one-third of the Democratic electorate. Despite that, Obama avoided a primary challenge and went on to win re-election.

Things looked even worse for President Bill Clinton. After losing 54 House and eight Senate seats in the 1994 midterm elections, many Democrats were open to a primary challenge. In a December 1994 Times-Mirror poll, 66 percent of Democrats said they "would like to see" a primary challenge. By anybody's reckoning, 66 percent is a big number.

Potential opponents smelled opportunity. "I think that people are going to look at the president in the next six to eight months, and they are going to make an assessment as to whether they believe that he can do the job," New Jersey Democratic Sen. Bill Bradley, a possible Clinton challenger, said in January 1995.

Clinton recognized the threat. "From the time [Clinton aide Harold Ickes] arrived at the White House at the beginning of [1994, he] was primarily concerned with avoiding a primary challenge," the Washington Post reported in December 1995. "Mr. Clinton, through prodigious fund raising and behind-the-scenes courting of the party base, had escaped a primary challenge," the paper reported in November 1996, after Clinton won re-election.

The president between Clinton and Obama, George W. Bush, did not face a primary challenge because of the special circumstances of his first term — the Sept. 11, 2001 terrorist attacks and the war in Iraq. Although many Democrats came to oppose the war by the 2004 election, Republicans were solidly with the president after the Iraq invasion in March 2003. Asked in a 2013 oral history project whether the Bush team worried about a primary challenge, campaign manager Ken Mehlman said, "We didn't think, if you looked at where the country was, and particularly where the right side of the electorate was in 2002 — I did not think that was a real risk."

Fast forward to today, after Republicans lost 41 House seats in the midterm elections. It is simply not surprising that some Republican voters would express an openness to see the president of their party, in this case Donald Trump, face a primary challenge. That's what voters do. On one hand, that reflects a measure of dissatisfaction with the president. On the other, it could be a peculiarity of polling; many people do not want to tell pollsters that their minds are closed so long before an election.

"At this early point in any campaign, people are not going to shut their minds," Republican pollster David Winston to me recently. "They just don't do that. There's a huge difference between people saying they're thinking through things and how they behave. My guess is those polls are probably true, but that's because the electorate is trying to be thoughtful."

It could be that, unlike Clinton, Bush, and Obama, Trump does end up facing a primary challenge. Such a challenge could be quite damaging, given that Presidents Ford, Carter, and Bush I all faced challenges and went on to lose the general election. Trump's 2016 margin of victory in some key states -- Wisconsin, Michigan, and Pennsylvania -- was quite narrow, so a challenge need not be all that powerful to do significant harm.

But most of all, the primary talk serves to emphasize that polls often don't mean much. Back in 2010, in reporting the number of Democrats who were open to an Obama challenge, the Associated Press added this caution: "Recent history shows presidents' early polling numbers mean little about their re-election prospects. At this stage two years before their re-elections, Presidents Bill Clinton and Ronald Reagan had approval ratings that were lower than Obama's now, according to the Gallup Poll; both men won a second term. The ratings for Presidents George H.W. Bush and Jimmy Carter were better than Obama's; both lost."