Or perhaps grant statehood to D.C. and Puerto Rico, it's only fair. Or divide a large state like California, this would give it more senators.
While Washington D.C. would be the most thoroughly Democratic state in the country (though it's questionable whether it's constitutional to make it into a state given its special status), Puerto Rico would most likely be a swing state.
The thing about Puerto Rico, though, is that it doesn't seem to particularly
want statehood on the whole. Pro-statehood proponents will happily point to referendums showing overwhelming support for statehood... what they don't mention is that those referendums only had about a quarter of the population vote in them because those opposing statehood boycotted it.
For the record, it should be further noted that the two parties in charge of Puerto Rico are not the Democrats and Republicans, but the PNP and PPD. Neither exactly map to the Republicans or Democrats. I do not think it's likely that the two political parties that have been running things for decades would suddenly vanish into the ether upon statehood and be replaced by Republicans and Democrats. More likely they'd stay in power and people elected to the House/Senate would be from those parties.
As for California, that's a bit of a gamble. You need to remember that while it is overall a liberal stronghold, California is actually rather mixed in terms of Democratic and Republican areas. It's just that the Democratic areas, most notably Los Angeles, are super-populated which allows them to control the state as a whole. Split up California the wrong way and you could end up with either no functional change (i.e. you get 1 Democrat and 1 Republican) or Republicans actually gaining Senators. Also, is this something California itself is okay with? The Constitution does specify that the state legislature needs to agree to being split up--I'm not sure how popular an idea that is in California itself.