At this point, we absolutely need to hear impeachment testimony from former national security adviser John Bolton. But conservatives and Democrats alike would both be wrong to mistake the ultrahawkish foreign policy expert for a totally impartial witness.

I agree with my colleague Tom Rogan that it is now vital to the integrity of the impeachment process to hear what Bolton has to say. That's because bombshell reports emerged this weekend based on Bolton’s book manuscript. He supposedly alleges that President Trump told him directly that he was indeed conditioning Ukrainian foreign aid on investigations into Democrats,

If true, this charge from Bolton offers the first substantial direct evidence that Trump was using the aid to pressure Ukraine's president — something his defenders ( foolishly ) still deny. It would go beyond the secondhand testimony we have heard so far and offer something more concrete than testimony such as that from U.S. Ambassador to the European Union Gordon Sondland, who concluded , but was never explicitly told, that there was horse-trading going on.

NEW - "But to not call Bolton as a witness would be at best an act of political cowardice. At worse, it would be a betrayal of senators' solemn oaths of office."https://t.co/pJWnjHfKqQ — Tom Rogan (@TomRtweets) January 27, 2020

For Bolton to say the president admitted to pressuring Ukraine would certainly change the game, even if it wouldn’t necessarily transform this charge into an impeachment-worthy offense for those, such as myself, who are skeptical that it actually warrants removal . Republican senators should nonetheless demand to hear witness testimony from Bolton in the interest of treating the allegations with the seriousness their office demands.

But let’s not pretend Bolton is completely neutral or free of bias in this whole affair.

Yes, he is well-qualified, respected, and considered a serious person across the conservative establishment. Yet Bolton is also a dedicated ideologue, with profound, sincere convictions stronger than almost anyone in public life that the United States must play a more aggressive and belligerent role in world affairs. Bolton wants to be involved in more military conflicts. He explicitly advocated for the launch of a preemptive invasion of North Korea, a hawkish, pro-intervention stance if there ever was one.

In this, Bolton represents the absolute antithesis of Trump, who thankfully sought diplomacy, not war, with North Korea. And as a candidate, Trump ran against the Iraq War , calling it “stupid” and a “horrible mistake.” Bolton was a chief apologist for and proponent of that war.

While the Trump administration’s execution of anti-interventionist foreign policy has certainly been mixed and somewhat unsatisfying — the U.S. has not yet withdrawn fully from Iraq or Afghanistan as Trump promised — there’s still no disputing that the president ran as an anti-war Republican and retains those instincts. His entire campaign was a repudiation of Bolton's ultrahawkish conservatism, which had become orthodoxy in the GOP establishment prior to Trump. And this has carried over into Trump’s presidency, at least somewhat, with him decrying “endless wars” vigorously in his 2019 State of the Union address.

Even after Trump appointed Bolton into his administration as national security adviser, the two continued to clash along these ideological lines.

This resulted in Bolton’s eventual firing, and after he was tossed out of the White House in September 2019, Trump blasted Bolton as “Mr. Tough Guy,” saying the adviser made “very big mistakes” and that his hawkish views weren’t in line with the "America First" foreign policy agenda. Meanwhile, Bolton has bashed the president’s foreign policy in private speeches and implied that Trump makes foreign policy decisions based on his personal interests.

Suffice it to say there’s clearly both massive ideological differences and personal animosity between the two.

None of this is to say that Bolton is lying about Ukraine — although one must at least note that this leak is very conveniently timed with his book release. Bolton is a serious person who deserves to be heard out by the Senate as it gives the impeachment charges their due consideration.

But let’s not overlook the fact that Bolton is likely biased against Trump. We should hear him out, but take any purely interpretive parts of his testimony with the grain of salt it deserves. After all, as an ultrahawk, Bolton has every ideological reason to want to see Trump’s "America First"-style Republicanism go down in flames.