Identify two instances, one positive and one negative, when the role of Christianity in American politics showed itself in an overt way during the twenty-first century.

The Christian right has been a powerful force in American politics for decades, often exerting pressure on politicians in order to encourage conservatism, but some of its key crusades, such as the one against abortion (currently legal in the US), have continued to be unsuccessful. In many cases, it could be argued that politicians use the power of the Christian right until they are elected but then fail to honor promises that were implied during the election period. George W. Bush, for example, took a hard stance against abortion during his election run and relied upon the support of Christian groups in order to become, and remain, President. However, once in power, he was unable to criminalize abortion, to the dismay of many of his supporters.
Answering this question really depends on your worldview and what you consider to be a "positive" or a "negative" in political terms, but we can identify some instances where the power of the Christian lobby has influenced government policy in the past two decades.
A clear-cut "positive," for example, might include the passing of the Fair Sentencing Act in 2011, which was supported by Christian conservative action groups. This law received bipartisan support in Congress after considerable lobbying from the Christian right.
For a "negative," we might look more closely into the presidency of George W. Bush, which was so dominated by the issue of the "war against terror" that Bush characterized it as a "holy war." Much has been written on the rise of Christian support for militarization after 9/11; there is a useful article linked below. But, in short, the support of the religious right for the evangelical George W. Bush put these Christian groups into closer proximity with actual power than they had experienced under previous presidents. Bush owed his Christian base, and he knew it. Bush's own Christian morality, combined with the growing support for militarization among Christian voters, swayed US foreign policy in the direction of active militarization after 9/11, and the "war on terror" framed the US as morally good. The US stood against the unchristian threat represented by the terrorists.
Of course, some do not view this militarization, the active role played by the US in this crusade, as a negative. Likewise, recent Christian support for Donald Trump and his immigration policies might be considered a positive by some. This article discusses the key role played by conservative Christians in his rise to power and the support for tight immigration policies found among some Christian groups. It is for you to determine whether the strong influence of Christianity in increasing the power of conservatism in the USA has been, in recent years, a positive or a negative.
https://digitalcommons.tacoma.uw.edu/cgi/viewcontent.cgi?article=1004&context=ppe_prize

https://www.theatlantic.com/politics/archive/2014/02/the-changing-face-of-christian-politics/283859/