Our Constitution distributes power broadly across three branches of government, and the federal, state, and local levels. Yet during presidential campaigns, candidates for offices across the country unite behind their party’s presidential nominee. This person becomes the representative of the entire coalition, and it is his or her responsibility to explain to voters what the party stands for.

Hillary Clinton may have many limitations as a candidate, but there's no doubt she's an able messenger for the Democrats. The GOP, by contrast, has selected Donald Trump, who lacks the capacity to make an argument on behalf of the conservative movement. Trump has effectively shifted this rhetorical burden to Republican officeholders, who lack the prominence to make a forceful, unified case for the party. As such, the two sides are grossly mismatched, with the Democrats holding a substantial advantage.

The difference was stark in the wake of the Orlando massacre. Clinton and congressional Democrats were quick to get on the same page—connecting the shooting to the need for more gun control. Clinton gave a high-profile speech that coincided with a legislative push by Senator Chris Murphy in the upper chamber, followed by a sit-in by liberals in the House of Representatives.

Ted Cruz, on the other hand, offered an expert rejoinder on the Senate floor, calling out Democrats for their carefully staged "political show" to transform a "terrorism issue" into a "gun control issue." He also wrote an op-ed arguing, "The events in Fort Hood, Boston, San Bernardino and now Orlando demonstrate that this administration has failed to produce a clear-eyed strategy to defeat Islamic-inspired terrorism." This is a strong and serious argument for Republicans to make, one focused on the failures of the president to keep the country safe.

But Cruz is not the nominee, so his cogent response received little attention. Instead, the spotlight was on Trump, whose oafish posturing left the GOP without a prominent spokesman to match Democratic rhetoric. Trump's first response was a self-congratulatory I-told-you-so tweet. Then he argued that club-goers should be allowed to carry weapons—a position the National Rifle Association's Wayne LaPierre was forced to rebuke. Trump further speculated about the need for racial profiling. Instead of focusing on the failures of Obama, as Cruz did, Trump veered from undisciplined to doltish. The Democrats won the argument by default.

What happens in an election where one side is led by a nominee who cannot make an argument on behalf of his party? The answer is: Nobody really knows. Until now, the major parties have nominated candidates who could communicate their basic messages to voters. Even those who lost by large margins—Barry Goldwater in 1964, George McGovern in 1972, Walter Mondale in 1984, Bob Dole in 1996—could at least make their parties' case, even if their salesmanship was less than impressive. Trump is a different species of candidate entirely. His typical argument is a mishmash of catchphrases and off-the-cuff bluster. And when Trump does stumble upon a coherent thought, it's likely to offend some crucial voting bloc.

Republican officials may be tempted to write Trump off and simply focus instead on down-ballot races. That assumes other candidates can effectively communicate the party's message while the GOP nominee is spouting gibberish and the Democrats are marching in lockstep behind Clinton. Will marginal Republican voters alienated by Trump turn out to support down-ballot GOP candidates? Will independents otherwise amenable to Republican arguments find themselves driven to vote for Clinton and down-ballot Democrats?

Unfortunately, there is evidence that the Trump effect is real, and working to the Democratic party's advantage. At the beginning of the year, the average of polls by Real Clear Politics had Obama's job approval at just 43 percent. Today it stands at 50 percent. The difference is not due to improvements in the economy, which remains so fragile the Fed has kept interest rates historically low. It is certainly not due to victories in the fight against terrorism—with the massacres in San Bernardino and Orlando, domestic allies of ISIS have killed scores of Americans. No, the difference is Trump, who is making Obama look good. The president's job approval numbers, for instance, jumped noticeably after Trump was asked in an interview about David Duke and the Ku Klux Klan and failed to denounce them. Obama's approval rating jumped again after Trump defeated Cruz in Indiana.

It is possible, of course, that down-ballot Republicans can mount effective campaigns without a decent nominee at the top of the ticket. Maybe they can "localize" their races. Maybe they can create enough space between themselves and Trump. Maybe they can persuade voters to elect them as a check on President Hillary Clinton. Recent polls from Quinnipiac University show senators Marco Rubio of Florida, Rob Portman of Ohio, and Pat Toomey of Pennsylvania holding their own in competitive races. But other data are more worrying: A Reuters-Ipsos generic ballot test showed congressional Democrats with a nine-point lead earlier this month. Trump's failing and flailing campaign may well drag down many Republicans who, given a normal nominee leading the ticket, would win.

There is still a chance to stop this impending debacle. The delegates who meet at the Republican National Convention in July have a great stake in the party's success in November, coming as they do from state and local party organizations. They can choose to unbind themselves from the primary and caucus results, and nominate a decent candidate who can make an intelligent case to voters in November.

Jay Cost is a staff writer at The Weekly Standard and the author of A Republic No More: Big Government and the Rise of American Political Corruption .