It starts with their favorite presidential candidate. It seems obvious (at least right now) that they will nominate Donald Trump. It doesn't seem to matter that he lost the last election by over 7 million votes. It also doesn't matter that he's been indicted by 3 grand juries for 78 crimes -- and may soon be indicted by a 4th grand jury.
They want to believe that all of the charges are just politically motivated. But voters disagree. A majority say Trump committed a crime by trying to overturn the 2020 election results. And an even larger majority believes he intentionally stole secret government documents (and shared them with people who didn't have a security clearance). Trump may get most Republican votes, but that's not enough to win. He would also need to get a substantial majority of Independent votes. But Independents are not going to vote for a candidate they consider to be a criminal.
A Trump loss could hurt down-ballot Republicans. But even if it didn't, those down-ballot Republicans are refusing to support what most voters want. Even worse, they are actually opposing those things.
Republicans, after the Supreme Court overturned Roe vs. Wade, have gone all in to ban abortion completely. And they would like to pass a nationwide abortion ban, including the use of the day-after pill. They don't care that the voters would like to see legal abortion (especially along the lines of Roe vs. Wade). A substantial majority wants that -- including in very red states (like Kansas and others). This issue has hurt Republicans since the overturning of Roe, and it will hurt them in 2024.
But that is just a start on the GOP craziness. They have already made it clear that they want to cut Social Security (even though it has never added a penny to the deficit or debt. And they want to cut Medicare. These two programs are the only thing keeping millions of seniors out of poverty, and able to get the medical care they need. They don't seem to care that Social Security and Medicare are very popular programs, with huge majorities of voters supporting them.
Significant majorities of voters also support a substantial raising of the national minimum wage. They know that $7.25 an hour is a poverty wage, and they don't think anyone willing to work hard at a full-time job should be forced to live in poverty. Republicans don't care. Most want to keep the minimum wage at a poverty level, and some would like to eliminate it altogether.
But while the Republicans don't care about poor and working people, they love the rich. They oppose making the rich (even the super-rich) pay their fair share of taxes. They want to cut taxes on the rich (and corporations) even more, even though most voters want the rich to pay more in taxes.
Most voters would also like to see an effective background check for people buying guns. They know that mass shootings and gun deaths are at record levels, and they want something done about it. But Republicans don't seem to care how many innocent people are killed. They don't want any restrictions on guns -- not even background checks (which are supported by at least 80% of the population).
I could go on, but you get the idea. Republicans are opposed to anything that will save lives and make life better for most Americans. They are hoping voters will overlook those items. Currently, they are trying to campaign just on immigration and the economy. The problem with that is undocumented immigration is currently lower than at any time during the Trump administration -- and the economy is steadily improving (even inflation), while a recession has been avoided.
They seem to think most voters like their right-wing craziness. They are wrong, and it's going to cost them dearly in the next election.