The irresistible force is the sense of discontent with how things have been going during this young century. Americans are displeased with a sluggish economy that fell into a deep recession and with foreign policies that seem to have produced disappointing results.
The immoveable object is the strong partisan polarization of American voters, who have been casting almost equal percentages of their votes for each political party in presidential and congressional elections for two decades now, except for a surge against Republicans in 2006 and 2008.
The irresistible force of discontent has been apparent in this year's primaries. Donald Trump, with 41 percent of the popular vote, has seized the Republican nomination. Bernie Sanders, with 42 percent of the popular vote, has moved the agenda of the Democratic Party significantly leftward.
Trump's victory raises the possibility that the immoveable object will be moved -- i.e., party alignments will shift. That is certainly suggested by the March and April polling that showed him behind Hillary Clinton by as much as 50 to 39 percent.
Trump's Republican opponents charged that his nomination could produce a landslide defeat like 1964 and 1972, when Barry Goldwater and George McGovern got 38 percent of the vote.
They worried in particular about the effect on Senate races, in which they're defending seven seats in states carried by Barack Obama in 2012. Obama won all but one of those states by relatively small margins, but the fear was that, in an era of straight ticket voting, a weak presidential nominee would pull down-ballot Republicans down to defeat.
The point about straight ticket voting is correct. Only 26 of 435 congressional districts voted for one party's presidential nominee and the other party's candidate for House of Representatives in 2012 -- the lowest number since 1920.
It's not obvious that voters have lost the capacity to split their tickets. Perhaps they just haven't seen the need to in years when presidential nominees and congressional candidates have been closely aligned. Given Trump's heterodox and rapidly changing platform, this year could be different.
That's consistent with primary season polling in target state Senate races, in which the numbers are roughly consistent with prevailing partisan patterns. Republican candidates could be hurt if Trump depresses Republican turnout. But as he has pointed out, accurately, Republican presidential primary turnout has been up this year, while Democratic primary turnout has been lower and is down from 2008.
Meanwhile, some recent polls suggest that Republican voters are coalescing around Trump. Quinnipiac has him running even or better with Clinton in Florida, Pennsylvania and Ohio, target states without whose electoral votes Barack Obama would have been defeated in 2012.
National polls by Reuters/Ipsos and Public Policy Polling show about equal percentages of Republicans supporting Trump and Democrats supporting Clinton, though Clinton's numbers may well rise after Bernie Sanders stops campaigning. Still, these are signs that Trump is consolidating Republican voters, perhaps more effectively than Republican elites.
In addition, polling after Trump's 50 percent-plus victories in six Northeastern primaries shows the general election race tightening, with Clinton leading him by an average margin of 47 to 41 percent.
You could argue that those are disappointing numbers for both candidates. The universally known Clinton is running behind Barack Obama's 51 percent majority and seems not to be benefiting from the recent small but possibly critical increase in Obama's job approval rating (from 46 percent in January to 49 percent in April). And the universally known Trump is running well below 46 percent, the lowest vote share for any major party nominee since 2000.
I have noticed something else that may be significant in recent polls: The number of undecided voters seems to be increasing -- rather than decreasing like it usually does when nominees are determined.
This could result from cross-pressures. Majorities of voters have unfavorable feelings toward both candidates, and probably a record share, about 25 percent, has unfavorable feelings toward both. Apparently, some voters are having trouble deciding which repellent candidate to vote for.
That's just one question still to be determined. Another is turnout. Will upscale suburbanites appalled by Trump or young people not enchanted with Clinton bother to vote?
So far the irresistible force doesn't seem to have moved the immoveable object as far as many people expected. But it's too early to say it won't.