The wider Joe Biden’s polling lead over Donald Trump becomes, the more excited Democrats get about not just securing the White House, but also the Senate.

At the start of the year, this would have seemed far-fetched. But the dramatic transformation of American politics, precipitated by the coronavirus and the related sharp economic downturn — as well as the choice of a moderate presidential candidate in the shape of Joe Biden — means that what was once seen as a long shot is now a very real possibility.

Here are the numbers.

Of the 35 Senate seats up for grabs in November, 12 are held by Democrats, 23 by Republicans. Democrats need to pick up four seats to be in the majority, but if they win the White House, then three would be enough for control because the Vice-President has the casting vote when the Senate is split.

Democrats should probably add one to those numbers given the likelihood that they lose a seat in Alabama. Doug Jones became the first Democrat to win in the very, very red state in decades in 2017, when he defeated Roy Moore in a special election in 2017. Now that he is up against a popular former college football coach, Tommy Tuberville, rather than an alleged child molester, his defeat seems likely.

Outside Alabama, however, the picture looks far rosier for Democrats — and really rather grim for the GOP.

According to the highly-regarded Cook Political Report’s classification of Senate races, just one other Democrat-held seat up for election this year is deemed competitive. Michigan Senator Gary Peters faces a formidable challenger in the shape of John James, a young black West Point graduate who flew Apache helicopter combat missions in Iraq. But even there, a new Fox News poll gives Peters a double-digit lead.

Meanwhile, the Cook Report classification deems nine Republican-held seats competitive; four are classified as toss-ups, four as “lean Republican” and one as “lean Democrat”.

The Republican senator in the deepest hole is Martha McSally in Arizona, where she faces a challenge from Mark Kelly, an astronaut and the husband of Gabby Giffords, a former Congresswoman who survived an assassination attempt in 2011. Kelly has raised twice as much money as McSally in a race more and more Republicans are writing off as a loss.

In Colorado, Democratic former governor and presidential also-ran John Hickenlooper held an 11-point lead in the most recent poll of his race against Republican incumbent Cory Gardner. In Maine, Susan Collins — a self-styled moderate — has not avoided a tough re-election battle by distancing herself from Trump. In Georgia, both Republican incumbents — Kelly Loeffler and David Perdue — face intriguing and tight fights for survival in once safe-seeming seats. One of the closest, and most expensive, races looks set to be in North Carolina, where there was nothing to separate the two candidates in the most recent poll. Iowa and Montana are two more states that could prove pivotal in deciding the future of the Senate.

The striking thing about the races where Republicans are on the back foot is how little they have in common. They are geographically disparate and demographically unconnected. They represent not a specific weakness for the party but a widespread problem. Given the increasing nationalisation of American politics, that problem stems first and foremost from the GOP’s increasingly unpopular presidential candidate.

In a presidential year, it has grown steadily harder for senators to defy the performance of the person at the top of ballot. In 2016, for the first time in modern American history, every state with a Senate election backed the same party for the presidency and the Senate. So-called ticket splitting, when voters opt for candidates from different parties in different races, is a dying political habit.

The upshot of all of this is that Democratic excitement about taking the Senate is justified. It also means an interesting disconnect has opened up between the narrow message of Biden’s campaign — a singular focus on Trump’s unfitness for office —  and the broad legislative potential of a Biden presidency. It’s a dynamic that might limit how big a lead he can open up, and even threatens to reverse the trend towards nationalisation, testing whether the link between presidential and senate votes can be broken. It could also make it harder for Biden to keep his party’s progressive wing happy as they push for a democratic mandate for their long policy wish list.

For millions of moderate voters, it is one thing to want to see the back of Trump, but quite another to write a blank legislative cheque to a Democratic Party that has shifted a long way to the left in recent years. Increasingly, the best hope for incumbent Republicans facing defeat is that the closer we get to November, the more the focus shifts from Biden to his party’s legislative ambitions.