First, it will have nothing to do with Trump. Although I believe most GOP politico's do not like Trump, but they are afraid of his base and have learned the golden rule over the past 20 years, stay together and vote together for the GOP candidate no matter who he is!! This is especially important since the party base is not the majority and is getting smaller each year. The real reason trump will win is the total stupidity of those who call them selves Dems. While the GOP party sticks together, the Dems look at themselves as progressives or Black or moderates or young or older and not as a consolidated Dem party member. We saw it clearly in the 2016, when a Dem does not get the candidate who he sees as "one with my ideas", he will either vote for the GOP candidate in protest or just not vote. That kind of person got Trump elected in 2016 and will do so again in 2020. The Dem progressives are hinting that if they do not get the candidate of their choice they will again stay home or vote for trump. The other Dem groups are doing the same. This seems especially true of the Black voters who are saying if you so not pick one of ours, you lose our vote as it will be a sign of taking us for granted. So the Dems can not choose a candidate or candidates that meet all the criteria of all of their groups and so will lose voters and the election. The Dems learned nothing from 2016 while the GOP did.
What kind of person got Trump elected in 2016? Take these stats into consideration as one has to put Trump's election into the context of 2016. 38% of all Americans viewed Hillary favorably, 58% unfavorably. Trump, 36% of all Americans viewed him favorably, 60% unfavorably. Neither one was liked and more Americans didn't want neither one to become president than did. 48% of all Americans voted for Hillary, 46% voted for Trump, 6% or 9 million voters voted third party against both candidates. 2 point difference in favorable's and unfavorable's, 2 point win for Hillary.
What gave Trump the White House was the independent vote. 27% of independents viewed Hillary favorably, 70% unfavorably. Trump, 40% viewed him favorably, 57% unfavorably. Trump was able to translate the fact more independents disliked Hillary more than Trump into a 46-42 win of the independent vote. Yet 12% of independents voted third party against both major party candidates. Interesting enough CNN asked those who voted third party whom they would have voted for if there were only the two major party candidates, Trump and Clinton. 19% answered Trump, 16% Clinton, 65% said they would not have voted. That 65% is an interesting state. It means almost six million voters thought it important enough to go to the polls and vote against both major party candidates instead of staying home. Such was their disdain for both major party candidates.They had to officially register their vote against both Trump and Clinton.
Do more Republicans vote for their candidate than Democrats. Yes, except in 2016 when a higher percentage of Democrats voted for Hillary than the percentage of Republicans vote for Trump. Here are percentages of each major party and their vote for their candidate.
2016 89% of democrats voted for Hillary, 88% of Republicans for Trump
2012 92% of democrats voted for Obama, 93% of Republicans for Romney
2008 89% of democrats voted for Obama, 90% of Republicans for McCain
2004 89% of democrats voted for Kerry, 93% of Republicans for G.W. Bush
2000 87% of democrats voted for Gore, 91% of Republicans for G.W. Bush.
That's your 20 years. Now 20 years of party affiliation in presidential years. Percentage of the total electorate.
2000 Democrats 33%, Republicans 28%
2004 Democrats 33%, Republicans 30%
2008 Democrats 36%, Republicans 25%
2012 Democrats 33%, Republicans 25%
2016 Democrats 30%, Republicans 27%
Today or as of 1 May 2019, Democrats 31%, Republicans 30%
As for shrinking parties. Democrats hit their highest percentage of the total electorate since WWII in 1961 and again in 1964 at 51%. Since 1964 those who affiliate or identify with the Democratic Party has dropped to 30% today. Republicans hit their highest percentage of the total electorate of 38% in 1944. Since Eisenhower the Republican Party pretty much stayed in the upper 20's with a couple of years where they hit 30%. In fact the GOP was as low as 21% in 1975, 77, 79 and at 22% in 76 and 78 all following Watergate.