Make Puerto Rico a state and we have two more leftist Senators and several more leftist House members. Plus additional electoral votes for the Democrats in the presidential election. It'll be like adding a larger District of Columbia.
Residents of Puerto Rico have never voted for Democrats or Republicans in general elections, and your assumption that they would vote Democrat seems to be based on the fact that (mostly low-income) children and grandchildren of Puerto Ricans in NYC and Chicago vote Democrat for President. But have you noticed that the Irish and Italians and Poles and just about every other ethnicity in NYC and Chicago votes Democrat for President? While Florida Puerto Ricans (who are for the most part recent transplants from the Island) voted for Gore in 2000, they voted for Jeb in 2002 and (according to some polls) for W. in 2004 and seem poised to make the GOP their permanent home, since they are for the most part very culturally conservative and understand the importance of the War on Terror.
BTW, if Puerto Rico were to become a state, it would rank 25th in population and would elect 2 Senators and 6 Representatives. I have no idea who would get elected at first---probably likeable politicians who had previously run under the local party labels---but within a few years I think you'd find that Puerto Rican voters are very similar to those in Louisiana: Very conservative on social issues, more liberal on economic issues, very pro-military (despite calling for the closing of the Vieques bombing range, which was a NIMBY issue, and which only came up after a civilian guard died in a bombing accident) and very protectionist of local industries. Pollster Frank Luntz found that voters in Puerto Rico ranked as very conservative on every social issue that he asked about except the death penalty (where the Catholic tradition leads most people to oppose it, although not as much as they used to). If I had to guess, I would say that 20 years after Puerto Rico is admitted as a state it will have 1 Republican and 1 Democrat Senator and 3 Republican and 3 Democrat Representatives.