I am married to a US citizen. I am currently in the process of applying for a Green Card as a spouse. I would like to visit my husband, and I understand I may not be admitted into the USA based on the fact a current visa application is underway.
What I want to know is this: if I am refused entry into the US, does this affect my chances of securing my future greencard?
I understand I may not be admitted into the USA based on the fact a current visa application is underway.
A pending visa application is no bar to admission in a different status. The problem with the pending visa application is that it may imply some fact that is inconsistent with the other status. In particular, to visit as a "visitor for business or pleasure" (that is, in B-1 or B-2 status, or through the visa waiver program), you must have no intent to immigrate, and your pending application for an immigrant visa indicates an intention to immigrate.
This isn't as much of a problem as it might seem, because as long as you do not intend to immigrate during that particular visit, you can be admitted as a temporary visitor. Still, it's wise to proceed with caution because an immigration officer can always decide that you haven't been sufficiently convincing -- they might see your pending application to immigrate and infer that you are trying to shortcut the process.
But that's not what you're asking about; it is just context.
if I am refused entry into the US, does this affect my chances of securing my future green card?
This depends on why you were refused. If you are refused for using deception, for example, or because of something else that implicates grounds of inadmissibility, then yes, it could well affect your application for an immigrant visa. However, if you are refused admission because you are found to have immigrant intent, that will not affect your application to become an immigrant, because of course an applicant's intending to immigrate is not a reason to refuse an immigrant visa.
if I am refused entry into the US, does this affect my chances of securing my future greencard?
It depends on what happens after you are denied entry. If the officer allows you to withdraw your application for admission and voluntarily depart, and you depart according to those terms, you have not triggered a ban and it should not affect your ability to get an immigrant visa (assuming you have no other bans, of course). If the officer does not allow you to withdraw your application for admission, you will be "removed" through expedited removal proceedings, and that triggers a 5-year ban under INA 212(a)(9)(A)(i) (8 USC 1182(a)(9)(A)(i)). If you have this ban, and wish to get an immigrant visa during the 5 years of the ban, you would have to file I-212 for permission to reapply.
It is at the officer's discretion whether to allow you to withdraw your application for admission. In other words, they can remove you even if you wish to withdraw your application for admission and voluntarily depart. In practice, most people who are denied entry are allowed to withdraw their application for admission, if they don't have any serious violations on their record.
(One potential caveat I forgot to mention is that if you are traveling to the US from a preclearance airport, e.g. in Canada, Ireland, or Abu Dhabi, and you are denied entry to the US at the foreign preclearance airport, you cannot be removed from the US since you are not in the US. So in that case, I believe there is no risk of triggering this particular ban. Another potential exception is if you are seeking entry on the Visa Waiver Program and are denied entry and removed, you are not considered to have been "removed" for the purposes of the ban, according to 8 CFR 217.4(a)(3).)
