Some folks view the recent Bush push on illegal-to-legal immigration as a cynical campaign move to capture Hispanic voters, and no old pol in his right mind would argue the point. The Hispanic bloc is one of the big plums on the electoral tree, and both sides are eager to shake it loose. During the Clinton administration, it seemed the only way to get deported as an Hispanic illegal alien was if Castro demanded your return, and the Dems actively worked to sign up illegals to vote (and still do--witness the combined effect of the "Motor Voter" law and the recent Dem push to get driver's license rights for illegals) while not enforcing the borders.
The Dem assumption (a correct one) is that illegals, especially new illegals, are more likely to vote Democrat. The Republican assumption is that the considerably larger legal and established Hispanic population can be won over to the GOP camp, also a view with some validity, and that loosening the laws to allow more guest worker visas for Hispanics will give their efforts a boost.
BUT...as always, there are other considerations lurking beyond the obvious, and a consideration of the demographics involved is revealing. Not the electoral demographics, but the statistics for birth rates. So, if you're one of those folks who simply can't abide the thought of neighbors that don't look just like you, now is the time to quit reading this and go hide under your covers.
It requires a birth rate (per woman) of slightly over 2.00 live births per lifetime to simply maintain population levels. (It must be slightly over 2.00 to cover deaths before parenthood in the childbearing age population). Our current rate is right at 2.06 births, which translates to an annual birth rate of 14.7 per 1000 population. This breaks down roughly as follows: White non-Hispanic 12.2; Black, non-Hispanic 17.9; Asian/Pacific Islander 16.7; and Hispanic 24.4. Percentages of total population, respectively, are 73.9, 12.1, 3.4, and 10.5. (I'm leaving out the "other" category right now to keep from bogging down in lists, but it's miniscule for our purposes).
Unless you've been living in a cave for the last couple of decades, you know that we have a demographic bulge of Baby Boomers heading into their retirement years, with the associated problem of a stretched Social Security system, too many people collecting as compared to currently paying in. These boomers are overwhelmingly white, non-Hispanic. The only cures for too many people collecting are to either  cut benefits,  raise taxes,  get more people paying into the system, or  some combination of these.
Since WNH's are, as a group, not even sustaining their population numbers through replacement births, it's pretty obvious that the only way to accomplish option 3 in whole or part is to either boost birth rates or working-age immigration or both. And the highest birth rates and immigration rates are among Hispanics. In short order Hispanics will outnumber blacks as a group. And another fact of immigration is that new immigrants are more likely to work their butts off for low wages, and to more rapidly advance through the socioeconomic strata than established resident groups, boosting the economy more in the process.
The trend of a growing Hispanic population percentage will happen regardless of our immigration laws, as already-settled legal Hispanics in the U.S. population are reproducing at faster rates than any other population group, and at twice the rate of WNH's. Like it or not, we WILL become an increasingly Hispanic nation. (Committed bigots should consider moving to Norway...or Idaho.) What both parties are trying to do is to speed up that process a bit to ameliorate the SS problem, while capturing as much of that segment of the electorate as possible.
Not that they'll ever admit it.