Conservatives supporting this curriculum change in Texas history textbooks clearly have a conservative agenda which includes a dominant Christian influence. Their attempt is to present the founding fathers as fundamentally Christian as well as stressing the impact of conservative politics; notably in the last century by including figures like Newt Gingrich.
But Clearly, the main reason they are opposed to Jefferson is that Jefferson is credited with establishing the precedent of separation of church and state. Although the First Amendment does not come right out and say this, it is implied as Jefferson believed religion is between the individual and God; not to be mediated or endorsed or limited by any government. Jefferson's letter to the Danbury Baptists in 1802 is the source most often quoted in this debate where, in support of the Danbury Baptists' right to establish their religion, he used the term 'wall of separation between church and state.' The First Amendment reads:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
Since the First Amendment implies that the government will not establish nor limit any religion, the wall Jefferson referred to in his letter is 'built' to maintain that religious freedom and to keep religious biases from intruding with political progress.
The Texas Board, openly conservative, endorsed by Don McLeroy, a self-described 'Christian Fundamentalist,' is making the point that not only is America moving to the right, but that it has been a conservative Christian nation since the Founding Fathers. The Texas Board is essentially ignoring the Constitution by attempting to introduce political bias and their brand of Christianity into public schools, which are a government institution.
Not to mention, their other agendas in science books (using I.D. to challenge Darwinism, where again, the matter is subject to debate, but the consensus is that this is another debate between religion and science; not science and science). Ergo, I.D. should be taught in a course on Theology or, at best, theoretical science/philosophy.