(CNN) Will Republicans ever get on board with impeaching and removing President Donald Trump from office? That's a question that we should be asking, given that Trump won't be removed unless a lot of Republican senators back such a move.

Republican lawmakers will likely only do it if a lot Republican voters are for it. History suggests that it will be a high, though not impossible, hill to climb.

Just 11% of Republicans favor impeaching and removing Trump from office in an average of polls taken since the House's impeachment inquiry began in September.

Still, that 11% is very close to the 12% of Republicans who favored impeaching and removing Richard Nixon from office in a Gallup poll conducted in late November and early December 1973, after the House Judiciary Committee started an impeachment inquiry into Nixon in late October 1973.

The Nixon episode is enlightening because it reveals that the numbers on impeaching and removing him among Republicans didn't move until later into the process. Less than 20% of Republicans were for impeaching and removing Nixon until the summer of 1974

By early August 1974, about 30% of Republicans were for impeaching and removing Nixon in both Gallup and Harris polling. Nixon would leave office that month.

Now, you could look at the Nixon saga and believe that the numbers may move on Trump late, too. Indeed, there are reasons to believe Trump's Republican numbers aren't rock solid. We've already seen Republican support for the inquiry jump from below 10% before it began to a little more than 15% in a current average of polls. (Support for the impeachment inquiry is higher across the political spectrum than the more far-reaching acts of impeaching and removing Trump from office.)

I'm not sure, however, it's as simple as seeing the numbers move on the inquiry and thinking they'll move on impeaching and removing, too.

Even if 30% of today's Senate Republicans were for removing Trump, that still wouldn't be enough to get Trump removed from office. The current makeup of the Senate is 47 in the Democratic caucus and 53 in the Republican caucus. You need 67 Senators to vote to remove Trump. Assuming all 47 in the Democratic caucus vote to remove Trump, you'd still need 20 members (or 37%) of the Republican caucus to vote to remove Trump. (Because of the makeup of the Senate in 1974, you'd have only needed 10 members of the Republican caucus -- or 23% -- to vote to remove Nixon, if every member of the Democratic caucus voted to remove .)

Further, take a look at the Republicans who were for impeaching Nixon back in 1974. I'm not aware of any polling that breaks down Republican impeachment support by ideology, but we can examine the seven House Judiciary Committee members who voted for at least one article of impeachment against Nixon. (There were 10 Republican members who voted against all of them.)

Now, ideology isn't everything and it's not easy to compare across eras, but it does tell us a lot about the current impeachment polling among Republicans.

As I noted last week , our CNN polling revealed moderate and liberal Republicans were both the group most in favor of impeaching and removing Trump as well as the group who had moved most on the issue. Nearly one-third of moderate/liberal Republicans were in favor of impeaching and removing Trump in late September. That was up from 16% in May. Just 1% of very conservative Republicans were, which was actually down from 2% in May. This very conservative Republican group is Trump's base.

This moderate/liberal part of the Republican Party still makes up about one-third of Republicans nationwide. But like in Congress, their percentage of the Republican electorate has dropped tremendously. Back in 1974, moderate/liberal Republicans made up about 50% of Republicans nationwide. On the other end, very conservatives have jumped from less than 10% in Nixon's era to about one-third of Republicans nationwide today.

The rise of the self-described very conservatives within Republican ranks is one big reason why it will be harder to get Republicans aboard the impeach-and-remove-Trump train than the impeach-and-remove-Nixon train.

It's difficult to see a lot of Republican senators moving unless very conservative voters in the electorate do. It's not going to need to be anywhere close to even 50% of very conservatives on board with impeaching and removing Trump for Republican senators to move. But it can't be only 1%.

We need to wait until we're further along in the impeachment process to know if there will be a shift among Republicans. If there is, very conservatives likely will be the last to move.