Current browser versions will get 'mad' (show a warning message) if you try to mix HTTP and HTTPS content on the same page. And most current browsers (and future versions, as announced) will show a warning message on a login page if the page is not SSL.
The only possible way to have a login field SSL'd is to put it in an iFrame that has SSL. And most browsers will complain about that (because you are mixing SSL and non-SSL content on a page).
Besides, if you are going to SSL a login field, you'll need an SSL certificate installed on your site. Since you have to do that, you might as well SSL the entire site.
There are several plugins that will SSL your site, ensuring that you don't have mixed content on a page. I've used the Really Simple SSL plugin https://wordpress.org/plugins/really-simple-ssl/ .
Get an SSL certificate from your hosting place. Install it, then use a plugin to get it working. The Really Simple SSL plugin works quite well and easily to do this.