Dem
51
GOP
49
This is one of those years in which the White House is 50-50, the Senate is 50-50, and the House is 50-50. Either party could end up with the trifecta, or there could be cohabitation, as might well happen in France on Sunday. If these things were independent, with a 0.5 chance of the Republicans winning any one of them, there would be only a 0.125 chance of them winning all of them. But they are not independent.
Ticket splitting has gone the way of the dodo. If more people show up to vote for Donald Trump than Joe Biden, there will probably be more votes for Republican Senate and House candidates than for Democratic ones, and vice versa. More votes nationally doesn't guarantee anything, because there are 51 separate races for the presidency, 34 races for the Senate, and 435 races for the House. Still, high turnout for either party is generally helpful up and down the line. Also, the same issues (abortion, immigration, etc.) often play out at all levels, which increases the correlation.
When Trump had the trifecta during the first two years of his presidency, he didn't get a lot of legislation through, largely due to infighting within the party and Trump's lack of knowledge about how things worked in the government. That won't happen again if he gets the trifecta. The Heritage Foundation's Project 2025 gives Trump a detailed plan of action and tells him what to do. The Heritage Foundation also has lists of potential candidates for many federal offices, so Trump will have loyal supporters in key positions at all levels.
Democrats know this and are scared to death of it. If, come October, the Democrats see that they are going to lose the White House, they might shift money from the presidential race to specific Senate or House races, in order to block the trifecta by winning one chamber of Congress. At this point, the House looks easier because with the certain loss of the West Virginia Senate seat, there is almost no path to get to 51 Senate seats. Realistically, the best case for the Democrats in the Senate is 50 seats and the vice presidency. In contrast, a clear majority of the House is realistic.
All in all, given a choice, it is better for a party to hold the Senate than the House. If a party holds the Senate, it can either confirm the president's nominees for executive and judicial positions, or block them. It is widely expected that if Trump wins, Justices Clarence Thomas (76) and Samuel Alito (74) will retire. If any Democrat wins, they will try to hold on for 4 more years. On the other hand, if an opposition party controls the House, they can have an impeach-a-thon, holding hearings on impeaching the president, cabinet officers, judges, and justices, before impeaching them. At the very least, such hearings can damage the president, even if none of them lead to convictions.
One sticky point that could come up if either party wins a narrow trifecta is what to do about the Senate filibuster. In no conceivable scenario will either party have 60 seats in the Senate. This means that the Senate minority party will be able to block pretty much everything except budget matters, which can be done by a simple majority using the reconciliation process. But getting the trifecta and then being able to do almost nothing except set tax levels will be very frustrating not only to party leaders but also to voters. They don't understand how Congress works and many will say: "We gave you complete control of the country, so how come you did nothing?" The temptation to limit or abolish the filibuster will be enormous if either side gets the trifecta. If neither party gets it, then the filibuster is safe for a while. (V)