If the GOP nominated an individual friendly to Hispanic voters who runs against a mediocre Democrat, the party could very well reach 40% of the Hispanic vote.
It's very debatable that
any Republican in a national race has done as well as 40% (maybe Eisenhower or someone from before records really exist). The official exits for '04 gave Bush 44% but that has been debunked in lot of places; as
this article points out, most experts believe it was closer to 35% to 37%.
Michael Dukakis - arguably the quintessential mediocre Democrat - got 70% of the Hispanic vote; Mondale got 66% (this is according to the Roper Center site). The Democrat who apparently did worst with Hispanics was Carter in '80, a very unpopular president in a three-way race who underperformed badly with a number of blocs (he barely won Jews and union members). And even then Reagan only got 37%.
So you basically need a 1980 situation (double-digit inflation, recession, Kennedy primary challenge, independent candidate, hostage crisis) or a 2004 rally-around-the-flag climate (which rarely benefits a challenger) to get the Republican marginally past 35%.
