Why is it that Senator Portman (who I am no fan of) changing his position is a sign of the party falling apart. But when President Obama also recently changed his position on gay marriage, that is not a sign of the democratic party falling apart. Seems like a curious question. It sounds like both are simply pandering to whichever way the wind is currently blowing.
Neither party is falling apart. In fact they both have quite the strangle hold on the political system. The last election was extremely close. More so than the 2008 election. There has been no great shift in the electorate for either party. Elections tend to depend on enthusiasm, which leads to voter turnout. And conservatives were not particularly sold on Mitt Romney or John McCain. As such Romney earned less votes nationally than John McCain by more than 2 million. The only reason the race was closer than it was in 2008 was President Obama lost even more votes by about 9 million from the prior election. In 2008 the President won 52% of the vote nationally. And the Republican Challenger won 45%. In 2012 the margin narrowed to about 50% for the President and 48% for the Republican Challenger. So while they republicans didn't win the Presidential race, they actually gained ground from the last election cycle among those that turned out. That's hardly a sign of self destructing.
All that is happening in the Republican party is more true conservatives like Rand Paul are helping to purge the party of the country club republicans that are no less big gov't bureaucrats than most of the democrats. And that is a healthy process for the party as far as I am concerned.